A new freezing solution approach has been designed that offers greater efficiency and sustainability compared to the classic freeze-drying process to make dust or superfine nanoparticles.
Study: Freezing Dissolution Method: A fast green technology to produce nanoparticles and ultrafine dust. Image credit: petrmalinak / Shutterstock.com
In research published in the journal ACS Sustainable Chemistry & Engineering, sphere-shaped ice particles formed in an aqueous mixture of NH4H2PO4 or NaHCO3 to produce their respective nanoparticles.
What is the lyophilization method?
Due to their important specific areas and strong reactivity, nanomaterials and superfine dusts are gaining popularity in fields such as sustainable and environmental applications.
Nanoparticles (NPs) and superfine dust are often produced by lyophilization techniques. The initial stage of the lyophilization technique is a cryogenic procedure that freezes particles or target molecules in an aqueous mixture.
In the aqueous mixture, the water molecules solidify rapidly by the rapid freezing stage, generating a crystallized ice frame. This step is also known as ice or freezing template. The crystallized ice frame forces dissolved molecules or components aimed at producing a nanoscale scaffolding architecture, which results in nanoscale or microscale pore substances.
The freezing stage defines the architecture of the scaffold and the ice template, as well as the crystal architecture of the target substances within the ice templates or scaffolds, depending on the freezing configuration.
The second phase is a drying procedure that uses the sublimation process to separate the water as ice insoles. The ice melts during the drying phase, but the target substances, particles or molecules remain inside the ice. From the inside of the ice, frozen NPs or porous substances with identical architecture and characteristics can be recovered.
Schematic diagram of the experimental configuration of the freeze-dissolving method (above) and the freeze-drying method (below). © Yu, Q., Wang, Y., Luo, J. and Yang, H. (2022).
Limitations of lyophilization
Due to the colder temperatures used in the drying phase, sublimation rates are slow and batch drying periods for common pharmacological articles can take up to several days. The production speeds of these batch-based technologies are limited by low freeze-drying speeds and extended cycle operating times.
Some drawbacks can be mitigated by buying a larger freezer dryer. Unfortunately, it takes much longer to establish a perfect vacuum setting, and the temperature and pressure are less consistent throughout the container, which could affect the output quality. As a result of the cold temperatures and the arrangement of the vacuum, the drying phase consumes a lot of energy.
How is the freezing solution method better?
The initial stage of freeze-drying is identical to that of freeze-drying, that is, freeze-drying to create ice that contains the target components and to build a target ice scaffolding architecture.
The ice is then dissolved at a cold temperature, such as a sub-zero temperature in an additional solvent with a low freezing point in the later phase of the freezing dissolution process. This additional solvent, such as ethanol, acts as an antisolvent for the target components, but shows miscibility with water.
As a result, the ice scaffold will dissipate rapidly into the additional solvent, leaving only the solid-state target components in the mixture, and the architecture of the target-produced components inside the ice will be preserved.
Fire-fighting chemicals, sodium bicarbonate, ammonium dihydrogen phosphate (NH4H2PO4) and sodium bicarbonate (NaHCO3) are soluble in water but do not dissolve in ethanol.
In this work, various amounts of sodium bicarbonate or ammonium dihydrogen phosphate, dissolved in water, were used to make NP using the freeze-dissolving technique, which was then evaluated with NP produced by lyophilization.
Schematic diagram of the lyophilization and lyophilization mechanisms for the formation and isolation of NaHCO3 nanoparticles. © Yu, Q., Wang, Y., Luo, J. and Yang, H. (2022).
Important discoveries
To extract superfine dust and NPs from ice templates within frozen particles, the proposed freeze-drying process offers greater efficiency and sustainability compared to the conventional freeze-drying approach.
The particles of aqueous mixtures of sodium bicarbonate and ammonium dihydrogen phosphate were quickly frozen to produce sphere-shaped ice particles, which were then filled with NP and superfine powder of NaHCO3 or NH4H2PO4.
The frozen components were dispersed in ethanol for 5 minutes at 10 ° C using the freezing solution procedure to separate the ice scaffold. The freeze-drying approach, on the other hand, took 1400 minutes to separate the ice scaffolding through the sublimation process. In identical experimental environments, the dimensions of the final products generated by the freezing solution approach were comparatively small compared to those produced by the freeze-drying approach.
The freeze-dissolve approach reported in this study is approximately 100 times faster and consumes approximately 100 times less energy compared to the freeze-drying approach, without the need for a large installation or a vacuum. As a result, the freezing solution process is likely to be used on an industrial scale with less time, energy, and footprint.
Reference
Yu, Q., Wang, Y., Luo, J. and Yang, H. (2022). Freezing Dissolution Method: A fast green technology to produce nanoparticles and ultrafine dust. ACS Sustainable Chemistry & Engineering. Available at: https://doi.org/10.1021/acssuschemeng.2c02270
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